1. Field of the Invention
The present invention relates to the communications field and, in particular, to a network processing node (e.g., MGW, MRFP) and method that can: (1) receive packets on a first heterogeneous link (e.g., wireless link); (2) manipulate the received packets based on known characteristics about a second heterogeneous link (e.g., “Internet” link); and (3) send the manipulated packets on the second heterogeneous link (e.g., “Internet” link).
2. Description of Related Art
The following abbreviations are herewith defined, at least some of which are referred to in the ensuing description of the prior art and the preferred embodiments of the present invention.    AMR Adaptive Multi Rate    AS Application Server    BW Bandwidth    BTS Base Transceiver Station    CSCF Call State Control Function    CMR Codec Mode Request    ECU Error Concealment Unit    EGPRS Enhanced General Packet Radio Service    FEC Forward Error Correction    FER Frame Erasure Rate    HSPA High Speed Packet Access    IMS IP Multimedia Subsystem    IP Internet Protocol    LAN Local Area Network    MDC Multiple Description Coding    MGW Media Gateway    MRFP Multimedia Resource Function Processor    NB Narrowband    RTP Real Time Protocol    SID Silence Descriptor    SIP Session Initiation Protocol    TCP Transmission Control Protocol    UE User Equipment    UDP User Datagram Protocol    VoIP Voice over IP    WB Wideband    WCDMA Wideband Code Division Multiple Access    WLAN Wireless LAN
In a communications network, channel coding redundancy is often added on top of source data to enable the robust transmission of the source data. In 1948, C. E. Shannon discussed channel coding when he demonstrated that by proper encoding of source data, errors induced by a communication link can be reduced without sacrificing the rate of information transfer. In particular, C. E. Shannon developed a separation theorem which states that source and channel coding can be designed separately to optimize the performance of a communications network assuming a block length is infinite and an entropy rate of the source data is less than the capacity of a time-invariant communications link. For a detailed description about C. E. Shannon's separation theorem, reference is made to the following document:                C. E. Shannon, “A mathematical theory of communications,” The Bell System Technical Journal, vol. 27, pp. 379-423 623-656, 1948.        
However, an infinite block length causes infinite delay and infinite complexity, which is not feasible in reality. And, communication links may also be time-varying and band-limited especially in a multi-user communication system. Thus, for optimal system design, the source data and channel coding needs to be jointly optimized. This is discussed in the following documents:                S. Vembu, S. Verdé, and Y. Steinberg, “When does the source-channel separation theorem hold?,” in Proc. IEEE Int. Symposium Inform. Theory, 27 Jun.-1 Jul. 1994, pp. 198.        S. Vembu, S. Verdú, and Y. Steinberg, “The source-channel separation theorem revisited,” IEEE Trans. Inform. Theory, vol. IT-41, no. 1, pp. 44-54, 1995.        K. Sayood, H. H. Otu, N. Demir, “Joint source/channel coding for variable length codes,” IEEE Trans. Commun., vol. COM-48, no. 5, pp. 787-794, 2000.In the last document, K. Sayood et al. classified joint source-channel coding into four classes:        
1. Joint source channel coders where source and channel coders are truly integrated (i.e., channel-optimized quantization). A detailed discussion about channel-optimized quantization is provided in the following document: R. M. Gray and D. L. Neuhoff, “Quantization,” IEEE Trans. Inform. Theory, vol. 44, no. 6, pp. 2325-2383, 1998.
2. Concatenated source channel coders where source and channel coders are cascaded and fixed total rates are divided into source and channel coders according to the channel condition (i.e., forward error correction (FEC)). A detailed discussion about FEC is provided in the following document: B. Sklar and F. J. Harris, “The ABCs of linear block codes,” IEEE Signal Processing Magazine, vol. 21, no. 4, pp. 14-35, 2004.
3. Unequal error protection where more bits are allocated to more important parts of the source data (i.e., uneven level protection). A detailed discussion about uneven level protection is provided in the following document: Li, F. Liu, J. Villasenor, J. Park, D. Park, and Y. Lee, “An RTP payload format for generic FEC with uneven level protection,” Internet Draft draft-ietf-avt-ulp-04.txt, Feb. 2002.
4. Constrained source-channel coding where source encoders and decoders are modified depending on the channel condition (i.e., multiple description coding (MDC)). A detailed discussion about MDC is provided in the following documents: (1) A. El Gamal and T. M. Cover, “Achievable rates for multiple descriptions,” IEEE Trans. Inform. Theory, vol. IT-28, no. 6, pp. 851-857, 1982; (2) V. A. Vaishampayan, “Design of multiple description scalar quantizer,” IEEE Tran. Inform. Theory, vol. 39, no. 3, pp. 821-834, May 1993; and (3) V. K. Goyal, “Multiple description coding: compression meets the network,” IEEE Signal Processing Magazine, vol. 18, no. 5, pp. 74-93, September 2001.
In the past, joint source-channel coding has been successfully used in a homogeneous communications network (e.g., wireless link). It would be desirable to be able to efficiently implement joint-source channel coding in a heterogeneous communications network (e.g. wireless link merged with the Internet). Unfortunately, today the use of joint-source channel coding in a heterogeneous communications network is not very efficient as is discussed below with respect to FIGS. 1-3.
Referring to FIG. 1, there is a block diagram illustrating the basic components of an exemplary heterogeneous communications network 100. As shown, UE-1 102 (mobile unit 102) interacts via a wireless link 104 (channel A) with a BTS-1 106. The BTS-1 106 is connected to a MRFP-1 108 which is connected to a MRFP-2 110 via a link 112 (channel B) within the Internet 114. The MRFP-2 110 is connected to a BTS-2 116 which interacts via a wireless link 118 (channel C) with UE-2 120 (mobile unit 120). In this heterogeneous communications network 100, the characteristics of the three links 104, 112 and 118 are very different. The two wireless links 104 and 118 most likely have a reasonable low packet loss rate, especially if radio bearers optimized for real time services are used. On the other hand, the “Internet” link 112 can have a much higher packet loss rate. Unfortunately, today the two UEs 102 and 120 need to add the redundancy to handle the total packet loss rate for all three links 104, 112 and 118. The main drawback of this is that the redundancy added to combat the packet loss on the “Internet” link 112 also needs to be carried over the two wireless links 104 and 118 on which capacity is a scarce resource. This drawback is discussed in more detail below with respect to FIGS. 2 and 3 (PRIOR ART).
Referring to FIGS. 2 and 3 (PRIOR ART), there are respectively illustrated two heterogeneous communications networks 100a and 100b configured like the one shown in FIG. 1 which are used to help describe the main drawback of requiring the UEs 102 and 120 to add the redundancy to handle the total packet loss rate for all three links 108, 112 and 118. In FIG. 2 (PRIOR ART), assume channel A (wireless link 104), channel B (“Internet” link 112), a channel C (wireless link 118) have a FER of 1%, 7% and 1%, respectively. If UE-1 102 sends a packet 200 to UE-2 120, then UE-1 generates source data 202 and also redundancy A, redundancy B and redundancy C. However, redundancy A is only used on channel A and not used on channels B and C. And, redundancy B is only used on channel B and not used on channels A and C. Likewise, redundancy C is only used on channel C and not used on channels A and B. As can be seen, this scheme results in an inefficient use of redundancy transmission.
Referring now to FIG. 3 (PRIOR ART), an alternate solution is shown in which UE-1 102 generates a packet 300 with source data 302 and only one redundancy 304 which is transmitted over channels A, B, and C. In this example, assume that channels A, B, and C have a FER of 1%, 7%, and 1%, respectively. And, assume that UE-1 102 generates the redundancy 304 for the worst case scenario which is 7% in this example and sends it over channels A, B, and C. However, this amount of redundancy does not result in the efficient use of the capacity on the wireless channels A and C. Alternatively, if UE-1 102 generates the redundancy 304 for the average FER case ((1%+7%+1%)/3=3%), then the capacity for the wireless channels A and C will be higher than the previous scenario, but this amount of redundancy is not enough to recover the 7% FER in the channel B. Neither of these scenarios is desirable. Accordingly, there has been and is a need to address this shortcoming by effectively implementing joint-source channel coding (e.g., full redundancy, partial redundancy) in a heterogeneous communications network. This problem and other problems are solved by the present invention.